It is well known that a device that reduces the rate of fuel injection mass flow at the beginning of each injection event will help reduce undesirable NOx emissions in the combustion exhaust from a diesel engine. In addition, fuel injection rate shaping at the beginning of each injection event, or what is sometimes commonly referred to in the art as "pilot or pre-injection", can improve the combustion efficiency of the engine. In the case of a hydraulically actuated fuel injection system, an intensifier piston is acted upon by a high pressure actuation fluid, such as lubricating oil. The intensifier piston in turn drives a plunger to raise fuel pressure within a pressurization chamber above a threshold level that opens the needle check and allows fuel to exit the nozzle outlet of the injector. In most such prior art fuel injectors, the piston/plunger assembly is specifically designed to act as a single rigid unit. Because the injection mass flow rate is strongly related to the movement rate of the plunger. The rigid contact between the intensifier piston and the plunger renders it impossible to alter plunger movement rate independent from the intensifier piston.
While there exists a myriad of techniques for introducing rate shaping into an injector of this type, many of these techniques suffer from drawbacks rendering them unsuitable or otherwise impractical.
The present invention is directed to introducing some rate shaping into such an injector by permitting relative movement between the piston and plunger at the beginning of each injection event.